DESCRIPTION: (Applicant's Abstract) Substituted amphetamines (SA), such as methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA, ecstacy), are well known street drugs of abuse. Other members of this pharmacological class, such as fenfluramine, are used clinically in the treatment of various psychiatric disorders. While the SA sit at opposite ends of the use-abuse continuum, they share the ability to exert neurotoxicity in serotonin neurons. Two proteins, tryptophan hydroxylase (TPH) and the 5HT transporter, define neurons as serotonergic. The function of each of these important proteins is significantly diminished by the SA in the full course of their neurotoxicity. The broad, long-term objectives of this proposal are to determine the mechanism(s) by which SA are toxic to TPH, the initial and rate-limiting enzyme in the neurons through the reactive oxygen species (ROS) and/or nitric oxide (NO). Emerging lines of evidence point to ROS and NO as mediators of many of the effects of SA. However, it is not known if ROS and NO are responsible for the SA-induced toxicity to 5HT neurons, or if they exert any effects on TPH activity. Since ROS have been hypothesized to play causative roles in the inactivation of TPH after SA ingestion, it is important to establish whether TPH is effected by these species. This proposal will establish, through three specifi aims, whether ROS or NO alter TPH activity. Specific aim 1 will test the hypothesis that NO and the ROS superoxide radical, hydroxyl radical, hydrogen peroxide, and the peroxynitrite anion, inactivate brain TPH. Specific aim 2 will test the hypothesis that tetrahydrobiopterin, the essential cofactor for TPH in the brain, plays a role in ROS-and NO-induced inactivation of THP by making the enzyme more sensitive to these agents. Specific aim 3 has in virto and in vivo counterparts: to test the hypothesis that TPH can be protected from ROS and NO in vitro by appropriate scavengers; and to extent these studies by testing agents which block the production of NO or which scavenge ROS for protection of TPH from MDMA-induced inactivation in vivo. Increases in brain levels of BH4 will test the hypothesis that the TPH cofactor can modulate the in vivo inactivation of TPH caused by MDMA as it does in vitro with NO/ROS.